Date: Tue, 9 Apr 2024 18:11:34 +0200 From: Rocky Hotas <rockyhotas@tilde.team> To: freebsd-hackers@freebsd.org Subject: Re: Re: Kernel module: return a number from a device Message-ID: <i6jrmxhkijsfgphjezx7xfsnfjctuwok7pak4x7kto62mo35cm@har56vwmn4zq> In-Reply-To: <86r0fh5twn.fsf@ltc.des.dev> References: <oghsl7sgcl6p6ym3wwa3w56hn52cdophcr4jxau4ronxevy4ar@ux223whjcthj> <86r0fh5twn.fsf@ltc.des.dev>
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On apr 07 12:50, Dag-Erling Smørgrav wrote: [...] > > Furthermore, this won't only return one byte; rather, it will return one > byte _at a time_, very inefficiently. This is why cat appears to hang. > To truly only return one byte, you need to look at uio->uio_offset and > return 0 without calling uiomove(), signaling EOF, if it is non-zero. > > In summary, you should write rolld_read() as: > > uint8_t roll = arc4random() % d_size; > if (uio->uio_offset > 0) > return (0); > return (uiomove(&roll, 1, uio)); A massive thank you for all your suggestions, which were very clarifying about the way `uio' data can be used. > You can also use uiomove_frombuf(), which will take care of that check > for you. It's a bit overkill when you're only writing a single byte, > but if you wanted to output text instead of binary, you could use this: > > char roll[2]; > roll[0] = '0' + arc4random() % d_size; > roll[1] = '\n'; > return (uiomove_frombuf(roll, sizeof(roll), uio)); Yes, I guess this is probably the most efficient way to perform this operation. If anyone moving the first steps into this topic and into uio(9) is interested, here are: 1) a repository with three versions of this same module: <https://github.com/rockyhotas/freebsd_tests/tree/main/kernel_modules>; 2) a blog post with some comments about them: <https://rockyhotas.github.io/freebsd,/kernel,/modules/2024/04/08/freebsd-random-number-module.html>; Rocky
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